1. Technical Field
The present invention relates to a semiconductor device that is suitable for use in, for example, high frequency circuits in wireless communication apparatuses and a power amplifier that is constructed with the semiconductor device.
2. Background Art
In general, conventional semiconductor devices are known, in which a plurality of amplifier circuits that amplify high frequency signals are connected in parallel between an input terminal and an output terminal (for example, refer to Patent Documents 1 to 4). In such conventional art, each of the amplifier circuits is constructed with, for example, a bipolar transistor. In such case, the bipolar transistor may have a structure in which the collector is connected to the output terminal, the emitter is connected to a ground terminal, and high frequency signals input to the base are amplified. Moreover, in the conventional art, a ballast resistor may be connected to the base so as to suppress thermorunaway of the bipolar transistor.
Circuit structures are disclosed in Patent Documents 1 and 2, in which high frequency signals and a bias voltage are separately input to the base of a bipolar transistor, and a ballast resistor is connected to a path on the bias voltage side. In this case, since a ballast resistor is connected in parallel with a path for high frequency signals, the gain of the high frequency signals is not reduced.
On the other hand, a structure is disclosed in Patent Document 3, in which high frequency signals and a bias voltage are supplied together via a ballast resistor, and a capacitor is connected in parallel with the ballast resistor. Thus, in the conventional art in Patent Document 3, a decrease in the gain of high frequency signals in a frequency band in use is alleviated.
Moreover, a structure is disclosed in Patent Document 4, in which high frequency signals and a bias voltage are separately input to the base of a bipolar transistor, a ballast resistor is connected to a path on the bias voltage side, and additionally, a by-pass capacitor is connected between an input terminal and a bias terminal (a terminal for a bias voltage). In this case, distortion of high frequency signals is reduced by causing a part of the alternating current component of the base current to bypass the ballast resistor using the by-pass capacitor.
Patent Document 1: U.S. Pat. No. 5,629,648
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-196865
Patent Document 3: U.S. Pat. No. 5,321,279
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-324325
In these conventional examples, although thermal stability of a bipolar transistor can be achieved by use of a ballast resistor, stability against oscillation is not considered. On the other hand, a semiconductor element such as a bipolar transistor does not self-stabilize and is unstable in a wide range of frequencies. Moreover, the lower the frequency, the higher the gain of a semiconductor element. Thus, abnormal oscillation tends to occur at low frequencies.
In the semiconductor device described in Patent Document 1, a ballast resistor is connected in parallel with a path for high frequency signals. Thus, the ballast resistor does not contribute to stability against oscillation, and the bipolar transistor is unstable in a wide range of low frequencies.
In the semiconductor device described in Patent Document 2, a first resistor is connected in a path for high frequency signals, and a second resistor is connected in series with the first resistor in a path for a bias voltage. In this case, the first and second resistors function as ballast resistors, and abnormal oscillation of a bipolar transistor is suppressed by the first resistor, so that the stability can be improved. However, in Patent Document 2, although the stability in frequencies in use (high frequencies) is considered, a band of frequencies lower than the frequencies in use is not considered. The resistance of the first resistor is, for example, about one tenth of the resistance of the second resistor. Thus, for example, when the frequency, in use, of high frequency signals is 5 GHz, a bipolar transistor is unstable at low frequencies of 1 GHz or less.
On the other hand, when the resistance of the first resistor is increased, a voltage drop due to the base current increases, which limits the obtainable amount of increase in the current. For example, when a semiconductor device is used as a power amplifier, a necessary output power is obtained by increasing in the current in response to an increase in the input power. A problem exists in that, when the resistance of the first resistor is excessively increased, an increase in the output power is suppressed by suppression of an increase in the current. Similarly, such suppression of an increase in the output power also occurs when a capacitor is connected in parallel with the first resistor.
The semiconductor device described in Patent Document 3 has a structure in which a capacitor is connected in parallel with a ballast resistor, and high frequency signals and a bias voltage are supplied together. However, since a ballast resistor is provided to achieve the thermal stability, stability against oscillation at low frequencies cannot be achieved sufficiently when the resistance of the ballast resistor is set to a level necessary to achieve thermal stability. On the other hand, when the resistance of the ballast resistor is increased, a problem exists, as in the case in Patent Document 2, in that a voltage drop due to the base current excessively increases, so that an increase in the output power is suppressed.
The semiconductor device described in Patent Document 4 has a structure in which a capacitor is connected between an input terminal and a bias terminal to reduce distortion of high frequency signals. However, since the impedance of the capacitor depends on the frequency, a problem exists in that the distortion cannot be reduced in a wide band. Moreover, in the semiconductor device described in Patent Document 4, a ballast resistor is connected in parallel with a path for high frequency signals, as in the case in Patent Document 1. Thus, a problem exists in that a bipolar transistor is unstable in a wide band of low frequencies.